Tuning the excess charge and inverting the salt rejection hierarchy of polyelectrolyte multilayer membranes

Abstract

Abstract Polyelectrolyte multilayers (PEMs) coated on porous membrane supports have shown versatile opportunities for tailoring the resulting membrane characteristics. PDADMAC/PSS multilayers used as a separation layer deliver outstanding NF properties in terms of selectivities and permeances, as well as stability. Charge overcompensation of PDADMAC limits the possibilities of tuning the charge distribution in the PEM. We present a new membrane preparation methodology combining low ionic strength in the polycation and high ionic strength in the polyanion coating solutions, resulting in a PEM build-up with excess negative fixed charges. In contrast to PEMs build from only either low or high ionic strength in the PE coating solutions, this asymmetric approach results in a significant higher adsorption of PSS than PDADMAC for the PEMs, as proved by QCM measurements. The high negative excess charge of the new PEM hollow fiber membranes (PEMMs) leads to Na 2 SO 4 rejections of above 99%. More specifically, the salt rejection hierarchy is inverted, having higher rejections for the monovalent NaCl than for MgCl 2 . PEMMs with a high amount of negative excess charges are especially promising for anion rejection applications such as phosphate removal from effluents. The achieved high rejections for mono- and divalent phosphate ions and simultaneously high fluxes can compete and even outperform commercial NF membranes. Combining asymmetric low and high ionic strength in the PE coating solutions is therefore an effective way to control the net excess charge and its related salt rejection hierarchy.